System and method for a voltage controlled oscillator

ABSTRACT

In accordance with an embodiment, a voltage controlled oscillator (VCO) includes a VCO core having a plurality of transistors, a bias resistor coupled between collector terminals of the VCO core and a first supply node, and a varactor circuit coupled to emitter terminals of the VCO core. The bias resistor is configured to limit a self-bias condition of the VCO core.

TECHNICAL FIELD

The present disclosure relates generally to an electronic device, andmore particularly to a system and method for a voltage controlledoscillator (VCO).

BACKGROUND

Applications in the millimeter-wave frequency regime have gainedsignificant interest in the past few years due to the rapid advancementin low cost semiconductor technologies such as silicon germanium (SiGe)and fine geometry complementary metal-oxide semiconductor (CMOS)processes. Availability of high speed bipolar and metal-oxidesemiconductor (MOS) transistors has led to a growing demand forintegrated circuits for mm-wave applications at 60 GHz, 77 GHz, and 80GHz and also beyond 100 GHz. Such applications include, for example,automotive radar and multi-gigabit communication systems.

In some radar systems, the distance between the radar and a target isdetermined by transmitting a frequency modulated signal, receiving areflection of the frequency modulated signal, and determining a distancebased on a time delay and/or frequency difference between thetransmission and reception of the frequency modulated signal.Resolution, accuracy and sensitivity of the radar system may depend, inpart, on the phase noise performance and frequency agility of theradar's frequency generation circuitry, which generally includes an RFoscillator and circuitry that controls the frequency of the RFoscillator.

As the operating frequencies of RF systems continue to increase,however, the generation of signals at such high frequencies poses amajor challenge. Oscillators that operate at high frequencies may sufferfrom a poor phase noise performance that caused by 1/f and thermal noisein the devices that comprise the VCO. Phase noise may be furthercompromised by non-linear effects of self-bias conditions that affectthe gain and tuning characteristics of the VCO.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a voltage controlled oscillator (VCO)includes a VCO core having a plurality of transistors, a bias resistorcoupled between collector terminals of the VCO core and a first supplynode, and a varactor circuit coupled to emitter terminals of the VCOcore. The bias resistor is configured to limit a self-bias condition ofthe VCO core.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1 a-d illustrate the operation of an example automotive radarsystem, a schematic of a conventional VCO, and a performance of theconventional VCO;

FIGS. 2 a-b illustrate schematics of embodiment VCOs;

FIGS. 3 a-f illustrate performance results of embodiment VCOs;

FIG. 4 illustrates a block diagram of an embodiment method; and

FIG. 5 illustrates an embodiment radar system.

Corresponding numerals and symbols in different figures generally referto corresponding parts unless otherwise indicated. The figures are drawnto clearly illustrate the relevant aspects of the preferred embodimentsand are not necessarily drawn to scale. To more clearly illustratecertain embodiments, a letter indicating variations of the samestructure, material, or process step may follow a figure number.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, a system and method for a radarsystem, such as an automotive radar system. The invention may also beapplied to other systems and applications that use RF oscillators, suchas general radar systems and wireless communications systems.

In embodiment s of the present invention, a low phase-noise VCO utilizesa feedback resistor to reduce the effect of self-biasing duringoperation. This reduction of self-biasing also mitigates the distortionof the varactor tuning characteristic caused by modulation of thevaractor during operation of the VCO, thereby preventing unexpectedvariation of VCO gain that may increase phase noise within a frequencygeneration system such as a phase locked loop (PLL) or reduce thelocking range of the same PLL. Further phase noise rejection may beachieved by decoupling harmonics of the VCO core from the varactor.

FIG. 1 a illustrates an example automotive radar scenario 100 in whichautomobile 102 has automotive radar system 104. Automotive radar system104 transmits and receives, for example, a frequency modulatedcontinuous wave (FMCW) signal, and detects reflections of thistransmitted signal in order to determine a distance between automotiveradar system 104 and other vehicles or objects on the road. In theillustrated scenario, a large vehicle 106, such as a truck is closer toautomobile 102 then a small vehicle 108, such as a motorcycle. Undernormal operating conditions, the echo or reflection off large vehicle106 will be of a higher amplitude then the reflection off small vehicle108 because large vehicle 106 is both larger and closer than smallvehicle 108.

FIG. 1 b illustrates a graph 120 of received signal level versusreceived frequency for the scenario of FIG. 1 a. Signal level versusfrequency curve 122 corresponds to the received reflection from largevehicle 106 and the frequency f1 of signal level peak 130 corresponds tothe distance between automotive radar system 104 and large vehicle 106.Likewise, signal level versus frequency curve 126 corresponds to thereceived reflection from small vehicle 108 and the frequency F2 of thesignal level peak 132 corresponds to the distance between automotiveradar system 104 and small vehicle 108. Accordingly, the distancebetween large vehicle 106 and small vehicle 108 is proportional to theseparation between frequencies F1 and F2.

Along with the desired output signal, the phase noise of the radartransmitter is also transmitted and reflected. The phase noise reflectedfrom large vehicle 106 is represented as dashed line 124. As seen ingraph 120, phase noise 124 affects the ability of the radar to receivesignals reflected from small vehicle 108. The signal to noise ratiobetween signal level peak 132 due to small vehicle 108 and thecorresponding noise floor due to phase noise reflected from largevehicle 106 is represented as length 134. It can be seen from the graphof FIG. 1B, that phase noise affects the ability of automotive radarsystem 104 to discern small and distant objects. The higher the phasenoise of the radar transmitter, the less able the radar system is ableto discern small and distant objects.

FIG. 1 c illustrates conventional VCO 150 according to a “push-push”architecture. VCO includes VCO core 151 having transistors 153 andinductors 154, matching networks 152, varactors 158 and current source160. Transistors 153 are biased according to bias voltage Vbias, and thecapacitance of varactors 158 are tuned according to tuning voltageVtune. The frequency of oscillation of VCO 150 is approximately:

${f_{OSC} = \frac{1}{2\pi\sqrt{L_{154}C_{158}}}},$where L₁₅₄ is the inductance of inductor 154 and C₁₅₈ is the capacitanceof varactor 158. The output of VCO 150 is taken a Vout, which providesan output frequency of twice f_(OSC).

Varactor 158 may be implemented as a diode capacitance that is inverselyproportional to the voltage applied across its terminals. This decreasein applied voltage may be due to the increase of the width of thedepletion region in the reversed bias diode that causes a correspondingdecrease in its capacitance. An example relationship between the DCvaractor capacitance C_(varactor) with respect to tuning voltage isshown as curve 170 in FIG. 1 d. As shown, that C_(varactor) decreaseswith increasing voltage. During operation of the VCO, however, theactual voltage across varactor 158 various with time during eachoscillation cycle of the VCO, therefor causing the capacitance to varyduring each oscillation cycle of the VCO. Curves 174, 176 a, 176 b and176 c represent the variance in the applied tuning voltage acrossvaractor 158 due to the change in output of the VCO over a cycle. Curves176 a-c represent a small VCO amplitude and curve 174 represents a largeVCO amplitude. As shown, when the VCO produces a large amplituderepresented by curve 174, the capacitance of varactor 158 varies fromthe capacitance value C1 corresponding to point 173 on the curve 170 tocapacitance value C2 corresponding to point 175 on curve 170. The netpractical effect in this large signal behavior causes a change in theeffective tuning characteristic of varactor 158. These effective tuningcharacteristics are represented as C_(eff) in FIG. 1 c. Curve 178represents the effective tuning characteristic corresponding to smallamplitude curves 176 a-c, and curve 180 represents the effective tuningcharacteristic corresponding to large amplitude curve 174. As shown,curve 180 representing the C_(eff) for a large signal amplitude has a“knee” in its tuning characteristic. This “knee” may effect a reducedVCO gain that can affect the capability of a PLL to achieve frequencylock across the VCO tuning range, and can increase the phase noise whenthe VCO operates in high K_(VCO) tuning range.

FIG. 2 a illustrates the VCO 200 according to an embodiment of thepresent invention, which includes VCO core 202, varactor circuit 204containing varactors 230, and bias circuit 210. In an embodiment, VCOcore includes transistors 212, capacitors 214 and transmission lineelements 216. In an embodiment, VCO is configured to oscillate at afrequency between about 5 GHz and about 40 GHz, for example, about 20GHz. However, in alternative embodiments, other oscillation frequencyranges may be used. Transmission line elements 216 are sized in order toproduce an inductive impedance at the bases of transistors 212. Biasvoltages to the bases of transistors 212 are provided by bias circuit210 that is coupled to VCC via transmission line element 222. In anembodiment, transmission line element 222 is sized to be a quarterwavelength at twice the frequency of oscillation of the VCO 200. In someembodiments, bias voltage VBIAS is filtered via bias filtering network207 having transmission line element 240 and capacitor 242. In someembodiments, transmission line element 240 has a quarter wavelength ofabout four times the oscillation frequency of VCO 200.

The collectors of transistors 212 are coupled to VCC via transmissionline elements 218, feedback resistor 220 and transmission line element222. In an embodiment, transmission line elements 218 are sized in orderto maximize the signal swing. Feedback resistor 220, in someembodiments, mitigates the self-bias effect of high VCO amplitudesdistorting the tuning curve of varactors 230. For example, under lowtemperature and/or fast process corner conditions, the tendency of thegain and current of transistors 212 to increase, which results in alarger signal swing in the VCO and varactor node, is counterbalanced bythe effect of feedback resistor 220. An increase in bias current and/orsignal swing that results in increase current causes a correspondingvoltage drop across feedback resistor 220. This voltage drop reduces thesignal swing of VCO core 202 and therefore decreases the effects of theself-bias effect on the VCO tuning characteristic. Under hightemperature and/or slow process corner conditions, on the other hand,the tendency of the gain and current of transistors 212 to decreaselimits the occurrence of self-biasing, such that the additional voltagedrop across feedback resistor 220 is negligible and/or does not cause anappreciable decrease in VCO amplitude. In some embodiments, theresistance of feedback resistor is between about 5Ω and about 10Ω for abias current of about 20 mA. Alternatively, bias currents and otherresistance values for feedback resistor 220 may be used.

Varactor circuit 204 includes varactor elements 230, AC couplingcapacitors 228, series transmission line elements 232, and RF chokecircuits that include transmission line elements 234 and 236 andcapacitor 238. In some embodiments, tuning voltage V_(TUNE) is filteredvia bias filtering network 208 having transmission line element 244 andcapacitor 246. In some embodiments, transmission line element 240 has aquarter wavelength of about four times the oscillation frequency of VCO200. The combination of each RF choke circuit and transmission lineelement 232 may form an inductive voltage divider. In an embodiment, ACcoupling capacitors 228 allow varactors 230 to be biased based onapplied tuning voltage Vtune and reference voltage Vn1. Seriestransmission line elements 232 and AC coupling capacitors 228 forms aseries resonant circuit that allows the fundamental frequency ofoscillator pass to varactors while attenuating the harmonics of VCO 200.In some embodiments, series transmission line elements 232 may beimplemented using a transmission line having a length of about 400μ inone example. In another example, the length of series transmission lineelements 232 may be between about 100μ and about 500μ. It should beunderstood, however, that the length of series transmission lineelements 232 may be outside of this range depending on the embodimentand its particular specifications. In some alternative embodiments,series transmission line elements 232 may be implemented using aninductive element.

In an embodiment, the RF choke circuit that includes transmission lineelements 234, 236 and capacitor 238 produces a high impedance to theemitters of transistors 212 at about twice the oscillation frequency ofVCO 200, and provides a lower impedance at other harmonics of theoscillation frequency. By providing a lower impedance to oscillationharmonics via series transmission line element 232 and the RF chokecircuit, phase noise may be improved due because of reduced non-linearbehavior of the varactor.

Output V_(OUT) of VCO 200 is coupled to the emitters of transistors 212via transmission line elements 224 and 226 that isolate the VCO corefrom the output, thereby forcing the fundamental signal of the VCO toremain in the VCO core. This also improves the quality factor of theresonator and leads to better phase noise performance. The tail currentfor transistors 212 is provided by transmission line element 248 andresistor 250. In an embodiment, transmission line element 248 has aquarter wavelength at twice the frequency of oscillation of VCO 200.

It should be understood that, in some embodiments, the sizing oftransmission elements within VCO 200 may vary from the lengths andcorresponding wavelengths described above depending on the particularembodiment and its specifications.

FIG. 2 b illustrates VCO 260 according to a further embodiment of thepresent invention. VCO 260 is similar to VCO 200 shown in FIG. 2 a, withthe addition of output filter 274, and various alternativeimplementation details. In an embodiment, transmission line element 222is split into series transmission line elements 222 a and 222 b,transmission line element 248 is split into series transmission lineelements 248 a and 248 b, and transmission line elements 216 are splitinto series transmission line elements 216 a and 216 b. Likewise, theemitters of transistors 212 are coupled to common node N2 viatransmission line elements 266 a-d. In varactor circuit 204,transmission line elements 236 are coupled to ground via resistor 262and capacitor 264.

In an embodiment, bias circuit 210 provides bias voltage V_(BIAS) viatransmission lines elements 292 a-b, resistor 296, diode coupledtransistors 298 a-c and capacitor 291. The emitters of transistors 298a-c are coupled to ground via resistor 299. Output filter 274 is coupledbetween node N2 and output port V_(OUT). In one embodiment, filter 274has is a bandpass filter having a center frequency of about two timesthe oscillation frequency of VCO 260. Alternatively, other filter typesand center frequencies may be used. Filter 274 is implemented as an LCladder circuit having capacitors 276, 280, 284, 286 and 290, andinductors implemented using transmission line elements 278, 282 and 288.In further alternative embodiments, the inductors may be implementedusing discrete or on-chip inductors.

FIG. 3 a illustrates a series of curves that represent equivalentcapacitance over frequencies with respect to various embodimentarchitectures. Curve 302 represents the performance of a VCO havingfeedback resistor 220 but not series transmission line elements 232 andnot the RF choke made of transmission line elements 234, 236 andcapacitor 238; curve 304 represents the performance of a VCO havingfeedback resistor 220, and the RF choke made of transmission lineelements 234, 236 and capacitor 238, but not series transmission lineelements 232; and curve 306 represents the performance of a VCO havingfeedback resistor 220, the RF choke made of transmission line elements234, 236 and capacitor 238, and series transmission line elements 232.As can be seen, curve 306 has a higher effective capacitance at higherfrequencies and a lower effective capacitance at lower frequencies,thereby increasing the tuning range of the VCO.

FIG. 3 b illustrates a measured spectral plot of output V_(OUT) of VCO200 shown in FIG. 2 a. Here, the output frequency of VCO 200 is about 40GHz, which is twice the frequency of oscillation of the VCO of 20 GHz.FIG. 3 c illustrates a phase noise plot showing the phase noiseperformance of VCO 200. As shown, the phase noise is about −82.76 dBc/Hzat a 50 KHz offset, −90.66 dBc/Hz at a 100 KHz offset, −110.48 dBc/Hz ata 1 MHz offset, and −129.12 dBc/Hz at a 10 MHz offset.

FIG. 3 d illustrates plots of oscillation frequency with respect totuning voltage for an embodiment VCO. Trace 310 represents theoscillation frequency with respect to tuning voltage at 25° C. and trace312 represents the oscillation frequency with respect to tuning voltageat 130° C. FIG. 3 e illustrates plots of VCO gain (KVCO) with respect totuning voltage for the embodiment VCO characterized in FIG. 3 d. Trace316 represents the VCO gain with respect to tuning voltage at 25° C. andtrace 314 represents the oscillation frequency with respect to tuningvoltage at 130° C. FIG. 3 f illustrates the frequency of oscillationwith respect to supply voltage VCC over various tuning voltages at 25°C. Trace 320 represents the frequency of oscillation with a tuningvoltage of about 0 V; trace 322 represents the frequency of oscillationwith a tuning voltage of about 2.5 V; and trace 324 represents thefrequency of oscillation with a tuning voltage of about 5.0 V. It shouldbe understood that the plots of FIGS. 3 a-f illustrate the performanceof example embodiments. Other embodiment VCOs may perform differentlyfrom what is illustrated in FIGS. 3 a-f.

FIG. 4 illustrates a block diagram 400 of an embodiment method ofoperating a VCO. In step 402, a supply voltage is applied to a VCO corevia a resistor. In one embodiment, the VCO core and feedback resistormay be implemented using a circuit similar to VCO core 202 and feedbackresistor 220 shown in FIG. 2 a. In step 404, a self-bias condition maybe limited using this resistor. In some embodiments, limiting theself-bias condition may mitigate the effects of self-biasing on theeffective capacitance of the VCO. In step 406, the VCO is tuned byapplying a tuning voltage to a varactor, and in step 408, a signal pathbetween the VCO core and the varactor is provided at the frequency ofoscillation. In some embodiments, this signal path is implemented usinga series resonant circuit having an AC coupling capacitor and atransmission line element coupled between the VCO core and a runningnode, for example, as shown in FIG. 2 a with respect to AC couplingcapacitors 228 and series transmission line elements 232. In step 410,harmonics of the VCO are shunted to a reference node, such as ground,using, for example an RF choke circuit. By performing steps 408 and 410,harmonics of the VCO that are coupled onto the varactor are attenuated,thereby improving phase noise performance in some embodiments.

FIG. 5 illustrates single-chip radar transmission system 500 thatincludes upconverter 502, power amplifier 504 and frequency generationcircuit 506. As shown, upconverter 502 upconverts baseband signal BB toa higher frequency signal, which is then amplified by power amplifier504 and output on pin OUT. In some embodiments, baseband signal BB maybe a swept frequency or other signal type used in a radar system.Frequency generation circuit 506 produces local oscillator signal LObased on a reference frequency on pin REF that may be generated using,for example, a crystal oscillator. In an embodiment, frequencygeneration circuit 506 is implemented using a phase locked loop (PLL)having phase detector 512, loop filter 510, VCO 508 and divider 514. VCO508 may be implemented using embodiment VCOs described herein. It shouldbe appreciated that system 500 is just one of many examples ofembodiment systems that may utilize embodiment oscillators. Alternativesystems may include, for example, wireless and wire line communicationsystems, and other systems that use VCOs.

In accordance with an embodiment, a voltage controlled oscillator (VCO)includes a VCO core having a plurality of transistors, a bias resistorcoupled between collector terminals of the VCO core and a first supplynode, and a varactor circuit coupled to emitter terminals of the VCOcore. The bias resistor is configured to limit a self-bias condition ofthe VCO core.

In an embodiment, the varactor circuit includes a first capacitor havinga first terminal coupled to an emitter node of the VCO core, a firsttransmission line element having a first terminal coupled to a secondterminal of the first capacitor, a first varactor diode having a firstterminal coupled to a second terminal of the first transmission line anda second terminal coupled to a tuning node, an RF choke circuit coupledbetween a second terminal of the first capacitor and a second referencenode. The first transmission line may include a length of at least 100μm, the RF choke circuit may have a quarter wavelength at about twice afrequency of operation of the VCO. In some embodiments, the firsttransmission line and the RF choke form an inductive voltage divider.

In an embodiment, The VCO further includes a fourth transmission lineelement coupled between the tuning node and an input tuning terminal, athird capacitor coupled between the input tuning terminal and the secondreference node, a fifth transmission line element coupled between a basebias node of the VCO core and an output node of a bias circuit, and afourth capacitor coupled between the output node of the bias circuit andthe second reference node. The fourth transmission line element may havea quarter wavelength at about four times a frequency of operation of theVCO, and the fifth transmission line element may have a quarterwavelength at about four times the frequency of operation of the VCO.

In an embodiment, RF choke circuit includes a second transmission lineelement having a first terminal coupled to the first terminal of thefirst transmission line element, a third transmission line elementhaving a first node coupled to a second terminal of the secondtransmission line element, and a second capacitor coupled between thefirst node of the first transmission line and the second reference node.The VCO may include an output node coupled to the emitter terminals ofthe VCO core. In an embodiment, VCO comprises a frequency of operationbetween about 10 GHz and about 30 GHz.

In accordance with a further embodiment, a voltage controlled oscillator(VCO) includes a VCO core having a plurality of transistors, and avaractor circuit coupled to emitter terminals of the VCO core. Thevaractor circuit includes a first capacitor having a first terminalcoupled to an emitter node of the VCO core, a first transmission lineelement having a first terminal coupled to a second terminal of thefirst capacitor, a first varactor diode having a first terminal coupledto a second terminal of the first transmission line and a secondterminal coupled to a tuning node, and an RF choke circuit coupledbetween a second terminal of the first capacitor and a second referencenode.

In an embodiment, the RF choke circuit includes a second transmissionline element having a first terminal coupled to the first terminal ofthe first transmission line element, a third transmission line elementhaving a first node coupled to a second terminal of the secondtransmission line element, and a second capacitor coupled between thefirst node of the first transmission line and a second reference node.The first transmission line may have a length of at least 100 μm, andthe first transmission line element and the RF choke may form aninductive voltage divider. In some embodiments, the RF choke circuit hasa quarter wavelength at about twice a frequency of operation of the VCO.

In an embodiment, the VCO further includes a resistor coupled between acommon supply node of the VCO core and a power supply input terminal ofthe VCO. This resistor may have a resistance between about 1 ohm andabout 20 ohms. Alternatively, other values may be used. In someembodiment, the VCO further includes a fourth transmission line elementcoupled between the power supply input terminal and the resistor, suchthat the fourth transmission line element has a quarter wavelength ofabout two times a frequency of operation of the VCO.

In accordance with a further embodiment, a method of operating a voltagecontrolled oscillator (VCO) includes applying a supply voltage to a VCOcore via a resistor coupled to collector terminals of the VCO core,limiting a self-bias condition of the VCO core via the resistor, andtuning the VCO comprising applying a tuning voltage to a varactorcircuit coupled to emitter terminals of the VCO core.

The method may further includes providing a signal path at the frequencyof oscillation of the VCO between a tuning node and the emitterterminals of the VCO core using a first transmission line element, andcoupling harmonics of the VCO to a reference node via an RF chokecircuit coupled between the emitter terminals of the VCO core and thereference node. In some embodiments, the RF choke has a quarterwavelength of about two times the frequency of oscillation of the VCO.

The method may further include filtering the tuning node using a secondtransmission line element coupled between a tuning terminal of the VCOand the tuning node. The second transmission line element may have aquarter wavelength of about four times the frequency of oscillation ofthe VCO.

Advantages of embodiments of the present invention include ability toproduce generate a frequency having very low phase noise. A furtheradvantage includes, for example, a wide VCO tuning range.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription.

What is claimed is:
 1. A voltage controlled oscillator (VCO) comprising:a VCO core comprising a plurality of transistors; a bias resistorcoupled between collector terminals of the VCO core and a firstreference node, wherein the bias resistor is configured to limit aself-bias condition of the VCO core; and a varactor circuit coupled toemitter terminals of the VCO core, wherein the varactor circuitcomprises a first capacitor having a first terminal coupled to anemitter node of the VCO core, a first transmission line element having afirst terminal coupled to a second terminal of the first capacitor, afirst varactor diode having a first terminal coupled to a secondterminal of the first transmission line elememt and a second terminalcoupled to a tuning node, and an RF choke circuit coupled between asecond terminal of the first capacitor and a second reference node. 2.The VCO of claim 1, wherein: the first transmission line elementcomprises a length of at least 100 μm; and the RF choke circuit has aquarter wavelength at about twice a frequency of operation of the VCO.3. The VCO of claim 1, wherein the first transmission line element andthe RF choke form an inductive voltage divider.
 4. The VCO of claim 1,further comprising: a second transmission line element coupled betweenthe tuning node and an input tuning terminal; a second capacitor coupledbetween the input tuning terminal and the second reference node; a thirdtransmission line element coupled between a base bias node of the VCOcore and an output node of a bias circuit; and a third capacitor coupledbetween the output node of the bias circuit and the second referencenode.
 5. The VCO of claim 4, wherein: the second transmission lineelement has a quarter wavelength at about four times a frequency ofoperation of the VCO; and the third transmission line element has aquarter wavelength at about four times the frequency of operation of theVCO.
 6. The VCO of claim 1, wherein the RF choke circuit comprises: asecond transmission line element having a first terminal coupled to thefirst terminal of the first transmission line element; a thirdtransmission line element having a first node coupled to a secondterminal of the second transmission line element; and a second capacitorcoupled between the first node of the first transmission line elementand the second reference node.
 7. The VCO of claim 1, wherein the VCOcomprises an output node coupled to the emitter terminals of the VCOcore.
 8. The VCO of claim 1, wherein the VCO comprises a frequency ofoperation between about 10 GHz and about 30 GHz.
 9. A voltage controlledoscillator (VCO) comprising: a VCO core comprising a plurality oftransistors; and a varactor circuit coupled to emitter terminals of theVCO core, wherein the varactor circuit comprises a first capacitorhaving a first terminal coupled to an emitter node of the VCO core, afirst transmission line element having a first terminal coupled to asecond terminal of the first capacitor, a first varactor diode having afirst terminal coupled to a second terminal of the first transmissionline element and a second terminal coupled to a tuning node, and an RFchoke circuit coupled between a second terminal of the first capacitorand a second reference node.
 10. The VCO of claim 9, wherein the RFchoke circuit comprises: a second transmission line element having afirst terminal coupled to the first terminal of the first transmissionline element, a third transmission line element having a first nodecoupled to a second terminal of the second transmission line element,and a second capacitor coupled between the first node of the firsttransmission line element and a second reference node.
 11. The VCO ofclaim 9, wherein the first transmission line element comprises a lengthof at least 100 μm.
 12. The VCO of claim 9, wherein the firsttransmission line element and the RF choke form an inductive voltagedivider.
 13. The VCO of claim 9, wherein the RF choke circuit has aquarter wavelength at about twice a frequency of operation of the VCO.14. The VCO of claim 9, further comprising a resistor coupled between acommon supply node of the VCO core and a power supply input terminal ofthe VCO.
 15. The VCO of claim 14, wherein the resistor has a resistancebetween about 1 ohm and about 20 ohms.
 16. The VCO of claim 14, furthercomprising a second transmission line element coupled between the powersupply input terminal and the resistor, wherein the second transmissionline element has a quarter wavelength of about two times a frequency ofoperation of the VCO.
 17. A method of operating a voltage controlledoscillator (VCO), the method comprising: applying a supply voltage to aVCO core via a resistor coupled to collector terminals of the VCO core;limiting a self-bias condition of the VCO core via the resistor; tuningthe VCO comprising applying a tuning voltage to a varactor circuitcoupled to emitter terminals of the VCO core; providing a signal path ata frequency of oscillation of the VCO between a tuning node and theemitter terminals of the VCO core using a first transmission lineelement; and coupling harmonics of the VCO to a reference node via an RFchoke circuit coupled between the emitter terminals of the VCO core andthe reference node.
 18. The method of claim 17, wherein the RF choke hasa quarter wavelength of about two times the frequency of oscillation ofthe VCO.
 19. The method of claim 17, further comprising filtering thetuning node using a second transmission line element coupled between atuning terminal of the VCO and the tuning node.
 20. The method of claim19, wherein the second transmission line element has a quarterwavelength of about four times the frequency of oscillation of the VCO.